Impact parameter

In physics, the impact parameter b is defined as the perpendicular distance between the path of a projectile and the center of a potential field U(r) created by an object that the projectile is approaching (see diagram).

It is often referred to in nuclear physics (see Rutherford scattering) and in classical mechanics.

The impact parameter is related to the scattering angle θ by[1] where v∞ is the velocity of the projectile when it is far from the center, and rmin is its closest distance from the center.

[2] The simplest example illustrating the use of the impact parameter is in the case of scattering from a sphere.

Here, the object that the projectile is approaching is a hard sphere with radius

, the projectile misses the hard sphere.

[3] In high-energy nuclear physics — specifically, in colliding-beam experiments — collisions may be classified according to their impact parameter.

, where the colliding nuclei are viewed as hard spheres with radius

[citation needed] Because the color force has an extremely short range, it cannot couple quarks that are separated by much more than one nucleon's radius; hence, strong interactions are suppressed in peripheral and ultraperipheral collisions.

This means that final-state particle multiplicity (the total number of particles resulting from the collision), is typically greatest in the most central collisions, due to the partons involved having the greatest probability of interacting in some way.

Because UPCs typically produce only two to four final-state particles, they are also relatively "clean" when compared to central collisions, which may produce hundreds of particles per event.